To provide cellular wireless communication service, a wireless service provider typically operates a radio access network (RAN) that defines one or more wireless coverage areas in which access terminals can be served by the RAN and can thereby communicate with other access terminals and obtain connectivity with broader networks such as the public switched telephone network (PSTN) and the Internet.
A typical RAN may include one or more base transceiver stations (BTSs) (e.g., macro network cell towers and/or femtocells), each of which may radiate to define one or more wireless coverage areas such as cells and cell sectors in which wireless access terminals (ATs) can operate. Further, the RAN may include one or more base station controllers (BSCs), radio network controllers (RNCs) or the like, which may be integrated with or otherwise in communication with the BTSs and may include or be in communication with a switch or gateway that provides connectivity with one or more transport networks. Conveniently with this arrangement, a cell phone, personal digital assistant, wirelessly equipped computer, or other access terminal (whether or not actually operated by a user) that is positioned within coverage of the RAN can then communicate with a BTS and in turn, via the BTS, with other served devices or with other entities on the transport network.
In general, a RAN will communicate with served access terminals according to an agreed air interface protocol, examples of which include CDMA (E.G., 1xRTT or 1xEV-DO), iDEN, WiMAX, LTE, TDMA, AMPS, GSM, GPRS, UMTS, or EDGE, and others now known or later developed. Communications in the direction from the RAN to access terminals define a “forward link”, while those in the direction from access terminals to the RAN define a “reverse link”.
A typical air interface protocol will provide a mechanism to distinguish communications in one coverage area from those in adjacent coverage areas and to distinguish between communications within a given coverage area. Under some air interface protocols, for instance, each coverage area may have a coverage-area identifier that distinguishes the coverage area from adjacent coverage areas, and communications in the coverage area may designate or be encoded with that coverage-area identifier in order to distinguish the communications from those in adjacent coverage areas. Likewise, each air interface connection (e.g., communication channel or other assigned connection resource) in a coverage area may have by a radio-link identifier, and communications carried on that connection may designate or be encoded with that radio-link identifier in order to distinguish the communications from others in the coverage area.
For example, under the CDMA 1xRTT protocol, each sector has a locally unique pseudonoise offset (“PN offset”) that is used to encode communications in the sector in a manner that distinguishes from communications in adjacent sectors, and each sector defines various control channels and traffic channels that are each encoded with a respective “Walsh code”. As another example, under the CDMA 1xEV-DO protocol, each sector similarly has a PN offset that distinguishes communications in the sector from those in adjacent sectors, and each sector designates connections assigned to various access terminals by respective “MAC Indexes” (which may translate to Walsh codes similarly used to encode communications). Other examples are possible as well.
A RAN will typically broadcast a pilot signal respectively in each coverage area, to enable access terminals to detect and evaluate cellular coverage. Further, the pilot signal of each coverage area may embody or designate the coverage-area identifier, so that access terminals can determine which coverage area is emitting the pilot signal. Under CDMA, for instance, the RAN may broadcast in each sector a pilot signal encoded with the sector's PN offset, so that if an access terminal detects a pilot signal encoded with that PN offset, the access terminal may determine that the PN offset is the coverage-area identifier of the sector that is emitting the pilot signal.
In an “idle” or “dormant” state where an access terminal is not actively engaged in a call or other communication session, the access terminal may regularly monitor the strength (e.g., signal-to-noise ratio (SNR)) of various pilot signals in search of a strongest pilot signal and thus a best coverage area in which to operate. If and when the access terminal then seeks to initiate a communication session, the access terminal may send a connection request (e.g., origination request) on an access channel of the selected coverage area, requesting the RAN to assign or otherwise establish a connection for the session. In response, the RAN may then assign a particular radio-link identifier (e.g., traffic channel or connection identifier, such as Walsh code or MAC Index) to the access terminal to be used in the coverage area, thereby transitioning the access terminal to an “active” state.
In the active state, when the access terminal is operating with an assigned connection in a given coverage area, the access terminal may regularly monitor the strength (e.g., SNR) of the pilot signal in that coverage area and the strengths of the pilot signals in neighboring coverage areas. If the pilot signal from another coverage area becomes sufficiently stronger than the pilot signal from the current serving coverage area (e.g., as a result of the access terminal moving toward the adjacent coverage area), the access terminal may then engage in control channel signaling with the RAN to arrange for a handoff of the communication session from the current coverage area to the other coverage area.
Under certain air interface protocols, such as CDMA for instance, an access terminal can operate actively in more than one coverage area at a time. Such an arrangement helps when the access terminal passes through an area of overlap between two or more coverage areas, as the access terminal may then engage in a “soft handoff” process that involves switching to communicate in a new coverage area before discontinuing communication in a previous coverage area. Further, soft handoff provides other advantages, such as allowing the access terminal and/or RAN to combine together or select the best quality of communications carried out simultaneously in the multiple coverage areas.
To facilitate soft handoff, an access terminal may maintain in its memory an “active set” that lists the coverage areas in which the access terminal has an active connection, and the RAN may likewise maintain a record of the access terminal's active set and will communicate with the access terminal in each listed coverage area. The active set may designate each coverage area by its coverage-area identifier and may further designate the connection assigned to the access terminal in that coverage area by its radio-link identifier. Generally, an active set may be limited in size to some defined number of coverage areas, such as three or six for instance.
In practice, the RAN may also provide the access terminal with a “neighbor list” that lists coverage areas neighboring (adjacent to or otherwise nearby) those in the access terminal's active set, designating each neighboring coverage area by its coverage-area identifier. The access terminal may then regularly evaluate the strength of pilot signals emitted by each coverage area of its active set and the strength of pilot signals emitted by each coverage area listed in the neighbor list, as well as the strength of other (remaining) pilot signals that the access terminal detects even if not listed in the access terminal's active set or neighbor list. If the access terminal thereby detects a pilot signal that is sufficiently strong compared with the weakest of the access terminal's active set members, the access terminal may then engage in signaling with the RAN to arrange for soft handoff to the detected coverage area and perhaps removal of the weaker coverage area from the access terminal's active set.
When an access terminal begins a communication session, the access terminal's active set may consist of just the coverage area in which the access terminal sent its connection request. At that point, the BTS serving that coverage area may provide the access terminal with a neighbor list designating neighbors of that one coverage area. As the communication session proceeds, the access terminal may then detect other coverage areas and arrange for addition of those other coverage areas to its active set through soft handoff.
Alternatively, a session can be initiated through a process known as “channel assignment into soft handoff” (CASHO), where the access terminal begins the session in a soft handoff state (having multiple coverage areas in its active set) rather than transitioning to that state over time in the session. In the CASHO process, the access terminal identifies multiple candidate coverage areas to initially include in its active set and, when requesting a connection in a particular coverage area, the access terminal provides the serving BTS with a list of the identified coverage areas (each designated by coverage-area identifier). The RAN may then establish a connection for the access terminal respectively in each identified coverage area, so that the access terminal can begin the communication session in a soft handoff state, active in multiple coverage areas at once.
In addition, as an access terminal operates with an assigned connection in any given coverage area, the access terminal and RAN will typically engage in a power control process to control the transmission power used for communication on that connection in that coverage area. Optimally, this power control process will help to keep the communication strong enough to overcome interference from other communications in the coverage area and from topographical obstructions, and will also help to prevent the communication from becoming so strong that it would unduly interfere with other communications.
In practice, for example, the RAN may regularly monitor the SNR of traffic channel communications that the RAN receives from the access terminal in a given coverage area and compare the SNR to a power control setpoint. If the SNR is lower than the power control setpoint, then the RAN may send to the access terminal a power control command that directs the access terminal to decrease the access terminal's transmission power on the assigned connection in that coverage area. On the other hand, if the SNR is higher than the power control setpoint, then the RAN may send to the access terminal a power control command that directs the access terminal to increase the access terminal's transmission power on the assigned connection in the coverage area.
The RAN may provide these power control commands to the access terminal in various ways. For example, the RAN may provide a power control command to the access terminal by transmitting the power control command on a forward link power control channel and including with the command a specification of the access terminal's radio-link identifier (e.g., MAC Index) to designate which access terminal in the coverage area is to receive the power control command. Alternatively, the RAN may provide the power control command as an overhead message in a channel that otherwise designates or embodies the access terminal's assigned radio-link identifier, such as in a channel encoded with a Walsh code assigned to the access terminal.
When an access terminal is concurrently active in multiple coverage areas, the access terminal may adjust its transmission power up or down in all of its active set coverage areas in a coordinated manner, by considering power control commands that it receives in its various active set coverage areas. In some CDMA systems, for instance, an access terminal may be arranged to increase its transmission power in all of its active set sectors if the access terminal receives power-up commands in all of those sectors, but to decrease its transmission power in all of its active sectors if the access terminal receives a power-down command in any one or more of the sectors.
The rationale for this coordination of power control across active set sectors is that if the RAN sends a power-down command to the access terminal in any sector, that means the RAN is receiving communications in that sector strongly enough from the access terminal to permit the power decrease and to still allow communication to occur in that sector. Thus, even if decreasing reverse link power in the access terminal's other active set sectors would deteriorate the quality of the access terminal's communication in those other sectors, the quality in the sector that sent the power-down command should suffice to adequately carry the access terminal's communications to the RAN. Meanwhile, decreasing reverse link power in the access terminal's other active set sectors may help to reduce interference on the air interface generally.